Ed Pawlowski wrote:

Certification. Medical oxygen has to be certified to a certain
purity, welding does not.

How exactly can compressed oxygen be "impure" ?

Are some oxygen molecules more pure than other oxygen molecules?

Or does the Medical oxygen tank look nicer and cleaner than the Welding
oxygen tank?

You pay for that test and the potential liability that goes
along with it.

I think you pay more for medical and aviation O2 because the
consequences can be more expensive if there is a problem with the
product (the product being compressed oxygen). The product itself is no
more expensive or different or has any additional processing steps done
to it on the basis of it's sale in it's variously-labelled forms.